Conventional radiation therapy techniques include the use of Intensity Modulated Radiation Therapy (“IMRT”), Arc Therapy, Three-Dimensional Conformal Radiation Therapy (“3-D CRT”), Particle Therapy, or Brachytherapy. The use of IMRT, for example, allows a radiation oncologist to treat a patient from multiple angles while varying the shape and dose of a radiation beam and thereby providing greatly enhanced ability to deliver radiation to a region of interest while avoiding excess irradiation of nearby healthy tissue.
Various treatment planning optimization techniques exist for developing radiation fluence patterns for external beam radiation therapy treatment plans. Treatment planning starts typically with images of an area of interest (e.g., slices from a CT scan), a desired dose of radiation which is to be delivered to a region of interest, such as a tumor, and “organs-at-risk” (OAR), which represent healthy tissues that are adjacent to or near the area of interest. A portion of a patient's anatomy that is intended to receive a therapeutic prescribed dose is referred to as a “planning target volume” (PTV). Both the PTV and any OAR may have complex three-dimensional shapes adding to the difficulty of preparing a treatment plan.
A variety of algorithms have been developed to solve an “inverse problem” of devising and optimizing a three-dimensional treatment plan for irradiating a planning target volume from a variety of angles to deliver a desired radiation dose to a region of interest while minimizing irradiation of nearby tissue (e.g., an OAR). Conventional treatment planning software packages are designed to import 3-D images from a diagnostic imaging source, for example, x-ray computed tomography (CT) scans. CT is able to provide an accurate three-dimensional model of a volume of interest (e.g., tumor bearing portion of the body) generated from a collection of CT slices and, thereby, the volume requiring treatment can be visualized in three dimensions.
During radiotherapy planning, volumetric structures are delineated to be targeted or avoided with respect to the administered radiation dose. That is, the radiation source is positioned in a sequence calculated to deliver the radiation dose that as closely as possible conforms to the tumor requiring treatment, while avoiding exposure of nearby healthy tissue (e.g., OAR). Once the region of interest (e.g., tumor) has been defined, and the critical normally-functioning tissue volumes have been specified, the responsible radiation oncologist specifies a desired radiation dose to the PTV and the allowable dose to OARS. Guided by a treatment planner or medical physicist, the software then produces a treatment plan that attempts to meet clinical dosimetric objectives expressed in terms of “dose-volume relationships.” These dose-volume relationships range from simple single-valued metrics mean dose) to the three-dimensional dose matrix itself. One commonly used embodiment of a dose-volume relationship is the dose-volume histogram (DVH) that summarizes the frequency distribution of radiation doses in a particular volumetric structure (PTV or OAR).
However, the above methods allow a planner to change objective criteria and guide inverse planning algorithms to a case-by-case solution, which then undergoes clinical review by a radiation oncologist before a patient is treated. Thus, the evaluation criteria are very subjective and depend on a planner's level of experience and an amount of time the planner has in developing the plan. In addition, planners and reviewers often accept plans when further sparing of an OAR is possible.